Orthodontic treatment involves movement of malpositioned teeth to desired locations in the oral cavity. One common type of orthodontic treatment involves the use of small, slotted orthodontic appliances known as brackets. The brackets are fixed to the patient's teeth and an archwire is placed in the slot of each bracket. The archwire forms a track to guide movement of the teeth to desired locations. The ends of orthodontic archwires are often connected to small orthodontic appliances known as buccal tubes that are, in turn, secured to the patient's molar teeth. In many instances, a set of brackets, buccal tubes and an archwire is provided for each of the patient's upper and lower dental arches. The brackets, buccal tubes and archwires are commonly and collectively referred to as “braces”.
Orthodontic brackets that are adapted to be adhesively bonded to the patient's teeth can be placed and fixed to the teeth using either one of two techniques known as direct bonding and indirect bonding. Direct bonding techniques generally involve the serial placement of individual adhesive-coated orthodontic brackets onto a patient's tooth surface by an orthodontist. Orthodontic brackets can be manufactured with a layer or coating of orthodontic adhesive on the base of each bracket. Typically, one bracket at a time is placed onto a patient's tooth surface until all of the brackets required for treatment are placed on the teeth. Alternatively, a layer or coating of orthodontic adhesive can be applied to the base of each bracket by the orthodontist immediately before the bracket is placed onto a tooth surface. In direct orthodontic bonding, the layer or coating of orthodontic adhesive on the orthodontic appliance is not hardened until after the orthodontic appliance is placed on a tooth surface. The layer or coating of orthodontic adhesive does not have a contour that is a negative replica of the tooth surface until the adhesive has been placed in contact with the tooth surface. Direct bonding techniques have been used to place and fix a single orthodontic bracket or serially fix a plurality of orthodontic brackets in a patient's oral cavity.
Indirect bonding techniques generally involve the use of a placement device or transfer apparatus having a shape that matches the configuration of at least part of the patient's dental arch. One type of placement device includes a “bonding tray” and typically has a cavity for receiving a plurality of teeth simultaneously. A set of orthodontic brackets may be releasably connected to the bonding tray at certain, predetermined locations. When the tray connected to the orthodontic appliances is placed over the matching portions of the patient's dental arch, each orthodontic appliance can be positioned on the patient's teeth.
In particular conventional indirect bonding techniques, before the bonding tray is formed, the brackets may be fixed to a plurality of teeth of a replica model of the patient's dental arch. Typically, an orthodontic adhesive is applied to the orthodontic brackets, the brackets are pressed onto the replica teeth, and the orthodontic adhesive can cure to a fully hardened condition which may involve use of, an orthodontic curing light. This fully hardened orthodontic adhesive may remain on the orthodontic brackets when it is removed from the replica teeth and can serve as a “custom base” for bonding the brackets to the patient's teeth.
Methods of making indirect bonding trays by taking a negative impression of each of the patient's dental arches and then making a replica model from each negative impression have been largely replaced by three-dimensional scanning or imaging using optical technologies such as: confocal laser microscopy, active wavefront sampling, accordion fringe inferometry, and optical coherent tomography. This may be followed by the use of three-dimensional printing technologies to produce a replica model of the patient's dental arches, such as: fused deposition modeling and printing, selective laser melting or sintering, electron beam melting, or inkjet three-dimensional printing. The brackets may then be temporarily bonded to the three dimensionally printed replica model of the patient's dental arches.
In one conventional example in the production of the bonding tray, a first matrix material can be applied to the orthodontic brackets bonded to the replica model of the patient's dental arches. Preferably, the first matrix material contacts the occlusal, facial, gingival, mesial and distal sides of the brackets. Optionally, but not necessarily, a portion of the first matrix material also contacts sections of the facial sides of the replica teeth of the replica model that extend along the bracket bases. Preferably, the first matrix material has a relatively low viscosity before hardening to assure intimate contact between the first matrix material and each bracket. In this manner, the first matrix material can substantially penetrate in the various recesses, cavities and other structural feature of the plurality of brackets to establish a connection between the first matrix material and each of the plurality of brackets.
An example of a suitable first matrix material may be EMILUMA® brand silicone material from Shofu Dental Corporation. The first matrix material may have a viscosity before curing that is preferably less than about 80,000 centipoise (“cP”), more preferably less than about 25,000 cP and most preferably less than about 8,000 cP. Once hardened, the matrix material may have a tensile stress at 20 percent elongation (according to ASTM D 412) in the range of about 31,000 to about 496,000 Pascal (“Pa”), more preferably in the range of about 62,000 to about 248,000 Pa and most preferably in the range of about 112,000 to about 136,000 Pa, and has a tensile stress at 50 percent elongation that may be in the range of about 91,000 to about 1,460,000 Pa, more preferably in the range of about 183,000 to about 730,000 Pa and most preferably in the range of about 329,000 to about 402,000 Pa. An example of a suitable first matrix material would be a first matrix material having a tensile stress at 20 percent elongation of about 124,000 Pa and a tensile stress at 50 elongation of about 365,000 Pa.
Subsequently, a quantity of a second matrix material may be dispensed to contact the labial, occlusal and lingual surfaces of the replica teeth of the replica model, except in areas covered by the first matrix material. The second matrix material extends over and preferably completely surrounds the first matrix material. Optionally, the second matrix material extends over the distal ends of the first matrix material adjacent the model molar teeth. The second matrix material also preferably surrounds an occlusal stop member except for those regions of the occlusal stop member that are in contact with the arch model. In this embodiment, the occlusal stop member is spaced from the first matrix material and separated from the first matrix material by the second matrix material.
An example of a suitable second matrix material is MEMOSIL 2® brand vinyl polysiloxane material from Heraeus Kulzer, Inc. The second matrix material can have a viscosity before hardening that is preferably less than about 1,000,000 cP, more preferably less than about 100,000 cP and most preferably less than about 8,000 cP. Once hardened, the second matrix material has a tensile stress at 20 percent elongation (according to ASTM D 412) that may be in the range of about 0.4×106 Pa to about 6.5×106 Pascal, more preferably in the range of about 0.8×106 Pa to about 3.3×106 Pa and most preferably in the range of about 1.1×106 Pa to about 1.4×106 Pa, and may have a tensile stress at 50 percent elongation that is in the range of about 0.8×106 Pa to about 12.5×106 Pa, more preferably in the range of about 1.6×106 to about 6.2×106 Pa and most preferably in the range of about 2.8×106 Pa to about 3.4×106 Pa. An example of a suitable second matrix material would be a second matrix material having a tensile stress at 20 percent elongation of about 1.3×106 Pa and a tensile stress at 50 elongation of about 3.1×106 Pa.
In methods which employ a second matrix material, the second matrix material can have a composition that is different than the composition of the first matrix material, and after hardening exhibits a tensile stress at 20 percent elongation that is preferably greater than the tensile stress at 20 percent elongation that is exhibited by the first matrix material after hardening. Preferably, the second matrix material chemically bonds to the first matrix material with a relatively high bond strength.
A bonding tray may further include an occlusal stop member that is connected to the second matrix material. Because the occlusal stop member matches the shape of the corresponding cusp tips of the replica teeth, the occlusal stop member can be firmly seated on the arch model in such a manner that little, if any, relative lateral movement is possible between the occlusal stop member and the arch model in an occlusal reference plane. The mated relation of the bonding tray to the teeth also helps to reduce the amount of unintended lateral movement of the tray before such time as the appliances are firmly bonded to the patient's teeth.
The occlusal stop member may be relatively inflexible having a Shore A hardness that is greater than the Shore A hardness of either of the first matrix material or the second matrix material. Preferably, the occlusal stop member has a Shore A hardness that is greater than about 72, more preferably has a Shore A hardness that is greater than about 90, even more preferably has a Shore D hardness that is greater than about 60 and most preferably has a Shore D hardness that is greater than about 75. An example of a suitable hardness is 72 Shore A hardness.
A bonding tray, including the brackets, connected to at least one matrix material, and optionally a second matrix material and the occlusal stop member thus formed, can then be removed from the replica model. Excess material of the tray may be trimmed as desired before use.
Other methods in production of the bonding tray may avoid use of the replica model of the patient's dental arches. A virtual replica model may be generated by three-dimensional scanning or imaging data of the patient's dental arch. One or a plurality of brackets can be virtually positioned in correct orientations relative to the virtual replica model. An indirect bonding tray may then be produced by three-dimensional printing using one or more matrix materials. The three dimensionally printed bonding tray includes negative spaces disposed in correct orientations to correspondingly receive and correctly orient one or a plurality of brackets. In this method, only one matrix material may be used in production of the bonding tray. The matrix material used in the three-dimensional printing of bonding trays, when cured, may have a wide range of Shore A hardness. The Shore A hardness can be between about 50 to about 90 with a tensile strength of between about 1.0×106 Pa to about 5.0×106 Pa.
Regardless of the method used to produce the bonding tray, the patient's teeth that are to receive the brackets may be dried, treated with an etching solution and a bonding adhesive can be applied to the bonding pad of the brackets retained in the bonding tray or to selected areas of the patient's teeth. The bonding tray retaining the brackets can then be positioned over the corresponding teeth of the patient and seated. Since the shape of the cavity presented by the matrix material(s), and optionally the occlusal stop member, together match the shape of the underlying teeth, the plurality of brackets can be concurrently seated against the underlying teeth with the intention of matching the same locations corresponding to the previous respective positions of the plurality of teeth on the replica model or at which the brackets were mechanically disposed based on three dimensional imaging data. Once the bonding adhesive has hardened, the bonding tray can be carefully removed from the patient's dental arch. After the bonding tray has been released from the patient's dental arch, an archwire is placed in the archwire slots of the brackets and ligated in place.
Understandably, the precise position of the brackets within the bonding tray at the time the bonding tray concurrently seats the plurality of brackets against the corresponding plurality of underlying teeth is an important factor to ensure that the teeth move to their intended final positions. However, counterpoised to maintaining the position of the brackets within the bonding tray at these precise locations, the matrix material must be relatively soft to readily release from the plurality of brackets correspondingly bonded to the plurality of teeth in the dental arch. The first matrix material can therefore allow one or more of the brackets to reposition within or prematurely release from the bonding tray during release from the replica model or during bonding to the patient's teeth.
Accordingly, there would an advantage in a bracket having a configuration which resisted repositioning or premature release from the matrix material during production, release from the replica model, or during bonding to the patient's teeth.